A User Equipment (UE), also known as a mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication network, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between UEs, between a UE and a wire connected telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks. The wireless communication may comprise various communication services such as voice, messaging, packet data, video, broadcast, etc.
The UE may further be referred to as mobile telephone, cellular telephone, computer tablet or laptop with wireless capability, etc. The UE in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another UE or a server.
The wireless communication network covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station, e.g., a Radio Base Station (RBS) or Base Transceiver Station (BTS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and/or terminology used.
Sometimes, the expression “cell” may be used for denoting the radio network node itself. However, the cell may also in normal terminology be used for the geographical area where radio coverage is provided by the radio network node at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes may communicate over the air interface operating on radio frequencies with any UE within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the UE. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the UE to the radio network node.
Beyond 3G mobile communication systems, such as e.g., 3GPP LTE, offer high data rate in the downlink by employing multiple antenna systems utilising Multiple-Input and Multiple-Output (MIMO).
Massive MIMO is a recently emerged technology that uses large Antenna Arrays Systems (AAS) with individual transceivers to improve throughput of wireless communication systems. Massive MIMO may sometimes also be referred to as “very large MIMO system”, or “large-scale antenna system”.
Antenna arrays with large number of elements enable the increase in capacity by utilising spatial beam forming and spatial multiplexing. The benefit of these large arrays is the ability to spatially resolve and separate received and transmitted signals with very high resolution.
The resolution is determined by the number of antenna elements, and their spacing. Typically the number of transceivers may be as high as 10× the maximum rank of the system. The rank is defined as the total number of parallel (same time and frequency) transmissions, including both wanted and unwanted signals (i.e. interference). Massive MIMO is sometimes loosely defined as a system using comprising 100 or more transceivers.
Basically, the more antennas the transmitter/receiver is equipped with in massive MIMO, the more the possible signal paths, the better the performance in terms of data rate and link reliability. Advantages with massive MIMO comprise improved UE detection and reduced transmit power per UE, thanks to the high resolution of massive MIMO.
However, moving from single antenna systems towards massive MIMO systems creates new problems and challenges that need to be solved, in order to reap the benefits.
Pre-distortion is performed in order to minimize nonlinearity effects in an amplifier. For a single antenna system, it is possible to have heavy Digital Pre Distortion (DPD) on the antenna element. But for a Massive MIMO system, since there are lots of antennas, it is required to have a heavy DPD for each amplifier, which is expensive and energy consuming.
One way to circumvent this problem may be to lower the Peak to Average Ratio (PAPR) of the signals going through each element. The technology that is currently in use is called Clipping. The signal is simply cut off when a signal peak is exceeding a threshold level. The signal is then filtered to retain a more spectral limited shape. To be more precise, firstly, a peak identifier will detect one or more peaks from the input signal. Then a cancellation pulse will be calculated to cut off the peaks. The crest factor reduced signal then needs to be filtered to reduce the undesired frequency components which will contribute to Adjacent Channel Leakage Ratio (ACLR).
Clipping however introduces an error in the transmitted signal which makes the receiver experience an increased Error Vector Magnitude (EVM), a difference of the received signal to the ideal one, which in the worst case may result in an erroneous decoded symbol. Clipping also widens the signals spectrum by creating hard clips.
Clipping is quite crude since it cuts a signal once it goes above a threshold and this will affect the UEs which added up to this peak. They will experience EVM, a difference from the expected signal to what is received which in the worst case may result in an erroneous decoded symbol.
ACLR will put energy outside of the systems initial frequency band because the hard clipping has a wider frequency spectrum which cannot be smoothed out completely by filtering. There is a maximum allowed ACLR for a system. Clipping may also put unwanted energy inside the systems own frequency band that will also contribute to EVM.
It appears that massive MIMO requires further development of the digital pre-processing for becoming feasible for practical implementation.